playground: upgrade to Debian Buster
playground: upgrade to Debian Buster

Buster is the latest stable release of Debian. Although Stretch is still
covered by LTS, it's better to be up to date.

Updates golang/go#25224

Change-Id: If91b7f3de834a6683880a1f661ff8777f58c38e7
Reviewed-on: https://go-review.googlesource.com/c/playground/+/227348
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>

golang/playground,golang/playground,golang/playground